WO02/068205, WO02/074548, WO2004/043704 and WO2005/012442, and also corresponding patent applications claiming the same priority dates, including U.S. patent applications Ser. Nos. 10/344,393, 10/380,381 and 10/899,888 (the content of each of which is incorporated herein by reference), describe laser imaging and also materials that can be used for that purpose. Examples that are provided typically involve the use high energy lasers.
There are many attractions in using non-contact near-IR sources, in particular diode lasers, to generate images from coatings for applications such as variable information packaging. Favourable attributes of diode lasers such as economy, portability and ease of use, are attractive for current needs in the packaging industry, such as in-store labelling.
The use of ink formulations that incorporate materials which absorb radiation from far-IR to mid-IR sources such as heat (˜1 to 20 μm) and CO2 laser (˜10 μm), allows the production of coatings that will generate a distinct coloured image on exposure to this wavelength of energy but not near-IR sources. The use of ink formulations that incorporate materials which absorb radiation from near-IR sources such as diode lasers (˜1 μm), allows the production of coatings that will generate a distinct coloured image on exposure to near, mid or far-IR irradiation.
The use of carbazoles and related compounds in substrate marking is known. U.S. Pat. No. 3,936,307A discloses multilayer coating with reactive moieties, including electron donors, in each of separate layers. GB2196137A, JP63221086A and U.S. Pat. No. 5,811,369A also disclose heterogeneous compositions.
Derwent WPI, Week 197533 (19 Jul. 1975), XP002330401 (& JP50021087B), discloses sensitization of recording material by forming a charge-transfer complex between a N-vinyl compound and an organic halide. A radical-based colour-forming mechanism is apparently involved.